The field of the invention is electrical insulation and, more particularly, the present invention relates to insulation having resistance to tracking resulting from high voltage. While polymeric materials are used for insulating a wide variety of electrical apparatus, most compositions are not suitable for high voltage applications in contaminated atmospheres where moisture or fog, together with salts, dust particles and ionic pollution, cause leakage currents to flow across the surface of the insulation. This current causes a rise in temperature with consequent moisture evaporation and ultimately dry band formation. The electrical stress across these dry bands often exceeds the breakdown stress of the air-insulation interface, so that discharge or spark scintillation takes place. The spark temperature is extremely high, often 2,000.degree. C or higher, and the heat produced may be sufficient to cause degradation of the insulation surface with the ultimate formation of carbonaceous spots. These carbonaceous spots usually link up in dendritic fashion and the organic insulation fails by progressive creepage tracking.
Over the years many solutions to these problems have been proposed of which perhaps the most effective has been the incorporation of hydrated alumina, preferably the trihydrate, in fairly substantial quantities to, for example, butyl rubber, epoxy resins, especially of the cycloaliphatic type, and, more recently, to ethylene-propylene rubbers.
There have been several suggested modes of operation for the hydrated alumina but, whatever the correct mechanism, it is found in practice that polymeric materials containing large proportions of alumina trihydrate are substantially protected against tracking and usually fail only by progressive surface erosion.
The amount of alumina hydrate required to produce the anti-tracking effect is very high, however, and is usually in the region of so - 90% by weight of the entire insulation. In the case of polymers that can be shaped by moulding or extrusion, the high filler content causes the following undesirable characteristics:
1. During the shaping operation, which can involve temperatures up to 200.degree. C or higher, the alumina hydrate starts to lose some of its water of hydration, which at such temperatures produces steam, which in turn leads to porous products. This must be avoided at all costs, since any voids or holes in an insulation material may produce catastrophic failure by corona discharge erosion on the inside of the void, which ultimately enlarges until failure occurs. At sufficiently high voltages, failure is extremely rapid and may be complete in a few seconds. PA1 11. In the the case of articles that are cross-linked after the shaping operation, especially by the use of high energy radiation of, for example, .beta.-or .gamma.-rays, the radiation can cause radiolysis of the hydrate to occur such that water is produced. This water appears to stay absorbed in the polymer/filler mixture until subsequently heated, e.g. for expansion or distortion purposes or in service, when foaming occurs. Such a foam (if a lot of filler is radiolysed) or even the formation of a few small blisters has the same catastrophic effect as the porosity described in (1). PA1 111. In the case of heat-shrinkable articles, the heat required to operate the shrinking process at an economic rate is high enough to cause loss of the hydrated water. If the shrinking temperature is very high this loss of water may cause porosity, and, even if no porosity is produced, the loss of any water reduces the performance of the polymeric insulation under polluting conditions.
Thus it is highly desirable to eliminate or greatly to reduce the porosity or void formation or loss of water which occurs when using alumina hydrate loaded materials.